High-voltage fuse with self-supporting spring fuse link

ABSTRACT

A high-voltage fuse includes a self-supporting fuse ribbon fuse link, or fusible element, i.e., one that is not provided with, or wound around, a link supporting insulating mandrel. The fusible element is formed of resilient material, preferably relatively hard silver, and forms a loaded extension spring. The fusible element is bent at a plurality of points in zigzag fashion having planar sections between bends. These planar sections enclose acute angles close to 90*. The ribbon fuse link has a plurality of serially arranged perforations and each of the aforementioned planar sections encompasses a plurality of such perforations. Each of the plurality of points where the fuse link, or fusible element, is bent is coextensive with one of said plurality of perforations thereof.

Mar. 7, 1972 United States Patent Kozacka 9 2 n 7 3 3 S m N m u w L m w.. Am R m O m Se TC N m Er TF M 1 w m MN F3 4 9. 4 6 8 [54] HIGH-VOLTAGE FUSE WITH SELF- SUPPORTING SPRING FUSE LINK gall [73] Assignee: The Chase-Shawmut Company, Newbu- Attorney-Erwin Salzer ryport, Mass.

[22] Filed: Jan. 26, 1971 PAENTEnM/R um n 3,648,210

' snm 1 nf 2 INVENTOR;

BY, )MJA/wm WW ATTY.

FREDERICK J. KOZACKA PATENTEDMAR 71972 3.648210 snm 2 nr 2 FIG. 3o

- CURRENT VOLTS ACROSS FUSE CURRENT VOLTS ACROSS FUSE mvENToR: FREDERICK J. KOZACKA BY www 3M/v ATTY.

BACKGROUND OF THE INVENTION In high-voltage fuses it is desirable to eliminate the conventional mandrels upon which the fusible element, or the fusible elements, are generally wound. Dispensing with such mandrels results in a significant cost reduction, and in a significant increase of the space available for the pulverulent arcquenching filler, which is normally quartz sand. One of the ways of avoiding the need of supporting insulating mandrels for the fusible elements is to wind the latter in helical fonn and to use relatively hard, resilient silver for the perforated ribbons forming the fusible. elements so that the latter take the form of, or are in effect, self-supporting extension springs. The term self-supporting includes the period during which the high-voltage fuse is being assembled. Upon successful assembly of a fuse its fusible element is surrounded and supported by the pulverulent arc-quenching filler, and then the support of the fusible element is less of a problem than during the assembly period of the fuse. High-voltage fuses including fusible elements formed by helically wound ribbons of relatively hard silver which fonn extension springs, and which fuses dispense with supporting mandrels for their fusible elements are, however, in particular instances, subject to limitations and it is the object of this invention to remove these limitations.

Fuses having helically wound self-supporting ribbon fuse links are fully disclosed in the copending patent applications of Frederick J. Kozacka et al., Ser. No. 879,090 for High-Voltage Fuse Having Helically Wound Ribbon Fuse Link, filed Nov. 24, 1969, now U.S. Pat. No. 3,571,776 and Ser. No. 16,116 for High-Voltage Fuse Having A Plurality Of Helically Wound Ribbon Fuse Links, filed Mar. 3, 1970, now U.S. Pat. No. 3,571,775.

The axial spacing A between the terminal elements of a high-voltage fuse is a quantity which is given to the designer of the particular high-voltage fuse, i.e., it is a quantity which he is not free to select. The number of perforations per unit of length of the fusible element and the total length L of the latter are parameters whose selection depends largely upon the circuit voltage, and upon the average magnitude of the arc-voltage required to be generated, and the maximum permissible magnitude of the arc-voltage which cannot be exceeded. In view of these constraints helical extension spring fusible elements are ruled out in a number of instances. To be more specific, where L it may occur that the pitch of a helical extension spring fusible ribbon element becomes too small for electrical reasons, mechanical reasons, or both. If the pitch of a helical extension spring fusible element is relatively small, the spacing formed between the fulgurites which take the place of contiguous convolutions, or turns, of the fusible element may become too small to achieve the required dielectric recovery following blowing of the fuse. Furthermore, if the pitch of a helical extension spring fusible ribbon element is small it tends to lose its self-supporting ability which ability depends, inter alia, upon the preloading, or stretching and concomitant increase of the pitch of the fusible element. In such instances another than the helical geometry of the fusible element is indicated in order to achieve the desired self-supporting properties thereof.

SUMMARY OF THE INVENTION lt has been found that the limitations to which helical selfsupporting extension spring fusible elements of high-voltage fuses are subject can be avoided if the three following conditions are met concurrently:

l. The fusible element means include ribbons of a resilient material (such as relatively hard silver) having serially arranged perforations therein bent at a plurality of points to form a zigzag-shaped extension spring having planar sections enclosing acute angles.

2. Each of said planar sections encompasses a plurality of serially arranged perforations.

3. Each of said plurality of points where said fusible element means is bent is coextensive with one of said serially arranged perforations thereof.

BRIEF DESCRIPTION OF DRAWINGS structure midway between the ends thereof being broken.

away;

FIG. 2 is a section substantially along II-Il of FIG. l;

FIG. 3 is an isometric view of a portion of one of the fusible elements of the structure of FIGS. l and 2;

FIGS. 3a and 3b are a current-trace and a voltage-trace, respectively, illustrating the interrupting performance of a high-voltage fuse whose fusible elements embodied the above features l and 2, but not the above feature 3; and

FIGS. 4a and 4b are a current-trace and a voltage-trace, respectively, illustrating the interrupting performance of a high-voltage fuse whose fusible elements embodied the three above referred to features I to 3, inclusive.

DESCRIPTION OF PREFERRED EMBODIMENT In the drawings reference numeral l has been applied to indicate a tubular casing of electric insulating material, e.g., a laminate of glass-cloth-melamine. Casing l is closed by a pair of terminal plugs 2 inserted into, or press-fitted into, the ends thereof. Plugs 2 are held in position by steel pins 3 projecting transversely through casing l into terminal plugs 2. A pair of terminal caps, or ferrules, 4 is mounted on the outer surface of casing l. The axially inner end surfaces of ferrules 4 and the k axially outer surfaces of tenninal plugs 2 abut against each other. The fuse may be provided with additional means for conductively connecting each terminal cap 4 to the terminal plug immediately adjacent thereto, e.g., a screw 4a may project through the end surface of each ferrule 4, clamping the latter against the adjacent terminal plug 2. Casing l contains a pulverulent arc-quenching filler 5, preferably quartz sand. Four self-supporting ribbon fuse links or fusible elements 6 are arranged inside of casing 1 conductively interconnecting terminal plugs 2. Each fusible element means 6 includes, or is made up of, a ribbon of relatively hard silver which is relatively resilient, and bent at a plurality of points 6a to form a zigzag-shaped extension spring having planar sections 6b enclosing acute angles a, preferably slightly less than Each fusible element, spring or ribbon 6 has a plurality of serially arranged perforations 6c. Each planar section 6b encompasses a plurality of serially arranged perforations 6c. As shown in FIG. 3, there are two perforations 6c between each pair of bends 6a. This number can be increased, if necessary, or desirable. Each of the points 6a, where a bend occurs, is coextensive with one of said plurality of perforations 6c. The ends of fusible elements, extension springs or ribbons 6 are arranged over centers and screwed by means of screws 7 against the axially inner end surfaces of terminal plugs 2. A gap is formed between the head of each screw 7 and the axially inner end surface of the terminal plug 2 immediately adjacent to it. The width of this gap is equal to the thickness of ribbons 6, and may be filled with solder by capillary action. Such solder joints minimize the ohmic resistance between fusible elements 6 and terminal plugs 2, and preclude screws 7 from getting lose. Each fusible element 6 in the form of a zigzag-shaped extension spring is arranged in one of a plurality of radial, angularly displaced planes. This is clearly shown in FIG. 2. As shown in FIG. 2 each of the fusible elements 6 is arranged in one of four radial planes which are angularly displaced 90.

The zigzag configuration of fusible elements 6 avoids occurrence of excessive thermal stresses and consequent fatigue, minimizing such stresses by distributing the same in equal increments along the entire length of the fuse link. This is, howthe dielectric recovery following blowing of the fuse, and the smaller the dimensional stability ofthe spring structures 6, all other conditions being equal. In view of these circumstances the optimal value of the angle a is a matter of compromise between contradictory requirements. It has been found that the angle a should be acute and preferably in the order of, but only slightly less than, 90. Springs 6 are, therefore, formed by folding of silver ribbons at angles much less than 90 and then stretching the folded ribbons until their planar sections 6b enclose angles of slightly less than 90.

It will be apparent from the above that the arrangement of relatively long planar sections 6b between bends 6a, i.e., of planar sections of which each encompasses a plurality of serially arranged perforations 6c, tends to increase the spacing d between bends 6a, and hence to optimize the dielectric recovery of the fuse structure following blowing thereof.

There is another factor which is of critical importance in the structure under consideration. This is the point of bending silver ribbons 6. It is well known that fuse links may either be bent between points of reduced cross-sectional area, or necks, or at points of reduced cross-sectional area, or necks. Each way of bending normally has its advantages and disadvantages. In the instant fuse structure the point of bending is critical, and may determine as to whether a particular fuse fails, or successfully interrupts a faulted circuit. ln the case of the instant fuse structure each of the plurality of points where fusible elements 6 are bent ought to be coextensive with one of the serially arranged perforations of the fusible element. This is clearly shown in FIG. 3.

FIGS. 3a, 3b, 4a and 4b illustrate the criticality of this particular feature. FIGS. 3a, 3b, 4a and 4b refer to tests performed in the same circuit under identical fault conditions including identical fault angles. The fuses to which FIGS. 3a and 3b refer and the fuses to which FIGS. 4a and 4b refer were identical, except that in the former instance the bends 6a were between perforations 6c and in the latter instance the bends 6a were coextensive with perforations 6c. As shown in FIG. 3a the current decays slightly upon the time of arc inception but beings to rise again thereafter before it decays to zero. According to FIG. 4a the current decays from the time of arc inception almost linearly down to zero. As a result, in the case of FIG. 3a the arcing F't is relatively large and in the case of FIG. 4a the arcing F1 is relatively small. The voltage-trace of FIG. 3b is consistent with the current-trace of FIG. 3a and the voltage-trace of FIG. 4b is consistent with the current-trace of FIG. 4a.

In the structure which has been described above the zigzagshaped ribbon fuse links may be forrried instead of a relatively hard silver of an alloy of silver and copper, wherein the silver content by far exceeds the copper contenLfI'he ratio of silver to copper may be in the order of 85 to I5. Such an alloy is more resilient than hardened resilient silver.

It will be apparent from FIG. 3 that each planar section of the fuse link encompasses a pair of circular perforations situated between the ends of each such section, and further en- Compasses one-half of a circular perforation at each of the ends of such a section. The number of intermediate circular perforations should be at least two, and may be more than two.

I claim as my invention:

l. A current-limiting high-voltage fuse including:

a. a tubular casing of electric insulating material;

b. a pair of cylindrical terminal plugs inserted into the ends of said casing and closing said casing;

c. a pulverulent arc-quenching t'iller inside said casing; and

d. self-supporting ribbon fuse link means conductively interconnecting said pair of terminal plugs and submersed in said arc-quenching filler, said ribbon fuse link means being of a resilient metal bent at a plurality of points to zigzag shape and forming a loaded extension spring having planar sections enclosing acute angles, said ribbon f use link means .further having serially arranged erforations, each of said planar sections encompassing etween the ends thereof a plurality of said perforations, and each of said plurality of points where said fuse link means is bent being coextensive with one of said perforations.

2. A current-limiting fuse as specified in claim l wherein said resilient metal is relatively hard silver and wherein said planar sections enclose angles in the order of and not exceeding 3. A current-limiting fuse as specified in claim l wherein said resilient metal is an alloy of silver and copper wherein the silver content by far exceeds the copper content and wherein said planar sections enclose angles in the order ofand not exceeding 90.

4. A current-limiting fuse as specified in claim l including a plurality of zigzag-shaped extension springs wherein each zigzag-shaped extension spring is arranged in one of a plurality of radial, angularly displaced planes.

5. A current-limiting high-voltage fuse including:

a. a tubular casing of electric insulating material;

b. a Dair of cylindrical terminal plugs inserted into the ends of said casing and closing said casing;

c. a pulverulent arc-quenching filler inside said casing; and

d. self-supporting fuse link means inside said casing conductively interconnecting said pair of terminal plugs and submersed in said arc-quenching filler, said fuse link means including ribbons of a resilient metal having serially arranged perforations therein bent at a plurality of points each coextensive with one of said plurality of perforations to form a zigzag-shaped extension spring having a planar sections enclosing acute angles and each encompassing a plurality of serially arranged perforations.

6. A current-limiting fuse as specified in claim 5 wherein said perforations are circular and wherein said planar sections enclose angles of about 90 and each of said planar sections encompasses between the endsY thereof at least one complete pair of said circular perforations, each of said plurality -of points where said fuse link means is bent being coextensive with a diameter of one of said plurality of circular perforations so that each of said planar sections is terminated by one-half of one of said circular perforations.

7. A current-limiting fuse as specified in claim 5 wherein said ribbons have overcenter ends screwed against the axially inner end surfaces of said pair of terminal plugs and also soldered to the axially inner end surfaces of said pair of terminal plugs.

8. A fusible element for a current-limiting high-voltage fuse including a narrow strip of a resilient metal bent at a plurality of points to zigzag shape and having planar sections between the bends thereof, said strip being stretched to such an extent that said planar sections thereof enclose angles of about 90, said strip further having serially arranged circular perforations, each of said planar sections encompassing between the ends thereof at least a complete pair of said circular perforations, and each of said plurality of points where said strip is bent being coextensive with one of said circular perforations so that each of said planar sections is terminated by one-half of one of said circular perforations. 

1. A current-limiting high-voltage fuse including: a. a tubular casing of electric insulating material; b. a pair of cylindrical terminal plugs inserted into the ends of said casing and closing said casing; c. a pulverulent arc-quenching filler inside said casing; and d. self-supporting ribbon fuse link means conductively interconnecting said pair of terminal plugs and submersed in said arc-quenching filler, said ribbon fuse link means being of a resilient metal bent at a plurality of points to zigzag shape and forming a loaded extension spring having planar sections enclosing acute angles, said ribbon fuse link means further having serially arranged perforations, each of said planar sections encompassing between the ends thereof a plurality of said perforations, and each of said plurality of points where said fuse link means is bent being coextensive with one of said perforations.
 2. A current-limiting fuse as specified in claim 1 wherein said resilient metal is relatively hard silver and wherein said planar sections enclose angles in the order of and not exceeding 90*.
 3. A current-limiting fuse as specified in claim 1 wherein said resilient metal is an alloy of silver and copper wherein the silver content by far exceeds the copper content and wherein said planar sections enclose angles in the order of and not exceeding 90*.
 4. A current-limiting fuse as specified in claim 1 including a plurality of zigzag-shaped extension springs wherein each zigzag-shaped extension spring is arranged in one of a plurality of radial, angularly displaced planes.
 5. A current-limiting high-voltage fuse including: a. a tubular casing of electric insulating material; b. a pair of cylindrical terminal plugs inserted into the ends of said casing and closing said casing; c. a pulverulent arc-quenching filler inside said casing; and d. self-supporting fuse link means inside said casing conductively interconnecting said pair of terminal plugs and submersed in said arc-quenching filler, said fuse link means including ribbons of a resilient metal having serially arranged perforations therein bent at a plurality of points each coextensive with one of said plurality of perforations to form a zigzag-shaped extension spring having a planar sectiOns enclosing acute angles and each encompassing a plurality of serially arranged perforations.
 6. A current-limiting fuse as specified in claim 5 wherein said perforations are circular and wherein said planar sections enclose angles of about 90* and each of said planar sections encompasses between the ends thereof at least one complete pair of said circular perforations, each of said plurality of points where said fuse link means is bent being coextensive with a diameter of one of said plurality of circular perforations so that each of said planar sections is terminated by one-half of one of said circular perforations.
 7. A current-limiting fuse as specified in claim 5 wherein said ribbons have overcenter ends screwed against the axially inner end surfaces of said pair of terminal plugs and also soldered to the axially inner end surfaces of said pair of terminal plugs.
 8. A fusible element for a current-limiting high-voltage fuse including a narrow strip of a resilient metal bent at a plurality of points to zigzag shape and having planar sections between the bends thereof, said strip being stretched to such an extent that said planar sections thereof enclose angles of about 90*, said strip further having serially arranged circular perforations, each of said planar sections encompassing between the ends thereof at least a complete pair of said circular perforations, and each of said plurality of points where said strip is bent being coextensive with one of said circular perforations so that each of said planar sections is terminated by one-half of one of said circular perforations. 